(1) Field of the Invention
The present invention relates to a system and method for automatically controlling a vehicle speed to a desired cruise speed applicable to a vehicle in which an automatic transmission is mounted.
(2) Background of the Art
A Japanese Patent Application First Publication sho 57-121713 published July 29, 1982 and counterpart Ito, et al. U.S. Pat. No. 4,421,192 exemplifies one of previously proposed automatic vehicle speed controlling systems for vehicles in which automatic transmissions are mounted.
In the above-identified Japanese Patent Application First Publication, the automatic vehicle speed controlling system includes: (1) vehicle speed signal generating means for outputting a first signal indicative of the vehicle speed; (2) vehicle speed setting means for setting the vehicle speed at which the vehicle is desired to cruise; (3) set vehicle speed signal outputting means for holding the first signal of the vehicle speed signal generating means when the vehicle speed setting means outputs the set signal; (4) vehicle speed comparing means for comparing the output signal of the vehicle speed signal generating means with the output signal of the vehicle speed setting means and outputting a comparison signal according to the result of comparison; (5) throttle valve controlling means for controlling an open and close of the engine throttle valve according to the comparison signal of the vehicle speed comparing means; (6) engine driving means for driving the engine according to the open and close states of the throttle valve; (7) set vehicle speed maintaining detecting means for comparing a set value set according to a gear position of the automatic transmission, the gear position being determined according to the vehicle speed when the vehicle speed setting means sets the vehicle speed at the constant run with a value of the output comparison signal of the vehicle speed comparing means and outputting a second signal when the value of the comparison signal exceeds the set value; and (8) a shift-down circuit which shifts down the gear at the constant run when the second signal from the set vehicle speed maintaining detecting means is outputted.
Furthermore, in the vehicle speed controlling system, a timer is provided which outputs a third signal during a predetermined period of time in response to the second signal derived from the set vehicle speed maintaining detecting means and the shift-down circuit shifts down the gear at the constant run on condition that a logical OR between the second and third signals derived from the set vehicle speed maintaining detecting means and the timer.
In the way described above, the previously proposed automatic vehicle speed controlling system shifts down the gear irrespective of the opening angle of the throttle valve when an absolute value of the vehicle speed difference between the vehicle speed and set vehicle speed exceeds a predetermined vehicle speed difference during the constant run (cruise run). For example, during the run on a slope having a predetermined gradient, a drive signal is outputted to an OD canceling solenoid to carry out the shift down when the speed difference between the set vehicle speed and vehicle speed exceeds the predetermined speed difference. However, during the cruise run on such slopes as in many repetitions of ascending and descending slopes, the shift down is carried out immediately before the run upto a summit of one ascending slope and the shift down is continued during the subsequent descending slope. Therefore, due to an increased continuation of an engine torque, the vehicle speed is increased more than necessary so that a vehicle comfortability is reduced. In addition, a hunting of the vehicle speed change from an overdrive position to a third gear position occurs at an interval of time determined by the timer during the run on a long continued ascending slope. Consequently, the number of times shocks in the gear changes transmitted to the vehicle body occur is increased and, thereby, the vehicle comfortability is reduced.